7×24 Conference Hits on Key Data Center Issues

By Keith Lane

07/01/2006

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Data center owners, operators, designers and vendors recently converged in Orlando for the annual 7×24 Conference. While a disparate group, all had one question on their minds: How does the industry deal with server power loads starting to creep up to almost 20 kW per rack with a not-so-distant potential load of up to 50 or 60 kW per rack?

First, savvy designers must learn how to use computational fluid dynamic modeling. CFD, according to Suhas Patankar, Ph.D., who presented on the topic, can be employed to simulate airflow within mission-critical environments to ensure that all servers are being provided with enough cool air.

Improper or low CFM levels can cause serious heating issues for the racks. Servers, said Patankar, are typically arranged in "hot/cold aisle" configurations where cold air is typically pulled from a raised floor through the front of the server rack housing and exits near the top and rear of the rack. The front is the cold aisle and the back is the hot aisle. As the velocity of air slows down from the computer room air-conditioner (CRAC) units under the raised floor to the perforated tiles below the racks, the pressure changes. Oddly, according to Patankar, air moving through the tiles furthest away from the CRAC unit have the most pressure.

Improper or low CFM levels will cause cross-flow of the hot air back into the cold aisle. The importance of proper modeling of the air dynamics becomes more critical with the higher densities forecast for the near future. Patankar argued it is far better to try to effectively simulate the proposed installation prior to construction than to try to fix design issues after construction is complete.

An interesting observation on energy savings for data centers was proferred by EYP MCF's William Kosik, who noted that increasing the chilled-water temperature from 45°F to 55°F increases the supply air in a data center floor from 55°F to 65°F—in other words, allowing a near 25% reduction in kW per ton. These savings are, of course, not free; the return-air temperature after cooling the servers is higher. These higher temperatures will require better airflow management and good layout of hot/cold aisles within the data center floor. But with the sheer amount of energy utilized in a data center, energy-saving measures can provide significant monetary savings to the owner and operators.

In another outside-the-box HVAC design session, given by Ronald Johanning, P.E., of Affiliated Engineers, and Guy Hutchins and David Leather barrow, both of Trox AITCS, the presenters argued for a new cooling solution: CO 2 . In a typical air-cooled data center, such systems, they claimed, are often limited to a maximum cooling of only 5 to 8 kW per rack. Alternatively, water-based solutions, while slightly better, are still limited to less than 20 kW per rack. CO 2 , according to the presenters, can provide cooling for densities that exceed 20 kW per rack. In fact, the Trox representatives presented a case study where CO 2 was used to cool a rack with a density of 30 kW. There was even some discussion about optimizing this system to cool up to 40 to 50 watts per rack. Furthermore, the CO 2 proponents pointed out that the cooling source is electrically benign and the cooling system itself can utilize smaller pipe diameters. Adequate infrastructure and proper planning, of course, are equally critical.

One of the drivers increasing power/cooling densities is the industry's gravitation toward virtual servers, which provide for clustering of real servers. While on the surface there appears to be some consolidation of servers, the actual service duty of the servers can significantly increase, in some estimations, as much 10% to 90%. This increase in actual operating time or load factor of the server could exacerbate already intensified power and cooling issues.